1. Field of the Invention
The invention relates to the control of flue gas temperature, and specifically relates to a method and system for establishing a substantially-uniform temperature of a flue gas from a gas turbine.
2. Discussion of Prior Art
Gas turbines have traditionally been used to drive loads such as an electric generator to generate electric power. In operation, gas turbines compress air to an elevated pressure and combine that air with fuel in a combustion chamber. The fuel/air combination is combusted to produce a high-pressure, high-velocity flue gas that travels over a series of turbine blades to rotate a shaft before being exhausted from the gas turbine.
At least some of the fuel introduced into the combustion chamber is exhausted without being fully combusted, forming undesirable emissions such as oxides of nitrogen (generally abbreviated as NOx) for example, which can be harmful to the environment. To reduce the amount of such undesirable components in the flue gas emitted by gas turbines, the flue gas has been subjected to a process referred to as Selective Catalytic Reduction (“SCR”). The SCR process involves introducing the flue gas to a reactant in the presence of a catalyst to achieve a reduction of NOx into less-harmful products. The high temperatures of the flue gas to which the catalyst is exposed, however, can deactivate the catalyst over time, requiring frequent replacement of the catalyst to effectively reduce the amount of NOx emitted by the gas turbine. Another high temperature limit is the temperature at which the reducing agent such as ammonia is oxidized producing NOx. Reducing the tempering air addition reduces operational cost as long as catalyst life and other considerations are accounted for.
Previous attempts to cool the flue gas before being exposed to the catalyst have involved delivering a cooling agent to the flue gas upstream of the catalyst. Generally, a static and predetermined stream of the cooling agent has traditionally been injected into the flue gas during operation of the gas turbine, regardless of the operating conditions. However, modern gas turbines will operate under a variety of conditions, emitting a varying flue gas based on those conditions, requiring different levels of the cooling agent to effectively cool the flue gas. Simple cycle gas turbines do not have steam cooling circuits to cool the gas turbine exhaust gases before the NOx abatement catalyst system. Air addition is a common method to reduce the exhaust temperature to an acceptable value before the NOx catalyst system.